Directional radio transmitting system



June' 4, 1946. l

M. l.. MacADAM DIRECTIONAL RADIO TRANSMITTING SYSTEM Filed Nov. 20, 1942ggg l Patented June 4, 1946 UNITD STT S ATEN E DIRECTIQNAL RADIOTRANSMITTING SYSTEM Application November 20, 1942, Serial N o. 466,245

2 Claims. 1

This invention relates to a radio transmitting system employing twodirectional transmitting antennas suitably spaced apart and disposedparallel to each other on suitable fixed supports whereby a markeddirectional effect in the direction of the antennas or in any desireddirection within a certain angle on either side of the azimuth of theantennas may be readily secured. It is a feature of the invention thatthe direction in which the signal energy is propagated may be readilychanged without disturbing the conductors of which the antennas arecomposed.

In the illustrated embodiment of the invention, the antennas are rhombicantennas of the same size and similar in construction. When two suchantennas are supplied with signal currents in such a manner that thecurrent in one antenna is in phase with that in the other, the radiationelds of the antennas combine to produce a main resultant field which ismarkedly directional and extends in the azimuth of the antennas and inthe sense toward which the antennas are directed. It has been found thatif the phase of the current in one antenna is shifted slightly withreference to that in the other antenna, the resultant main radiation eldmay be shifted to either side of its original or center positiondepending upon the direction of the phase shift. It is a feature of thepresent invention that a control means is provided for securing such ashift in the resultant main radiation field with the result that theradiation eld may be directed exactly in the azi-- muth of the antennasor exactly onto a desired region lying Within a number of degrees oneither side of said azimuth. It has also been found that the controlmeans provided is effective even though the two rhombic antennas are notexactly similar in size and construction or if the transmission linesconnecting the antennas to the final amplifying stages of thetransmitter are not exactly similar; these variations in the two antennapaths being compensated for by shifting the phase of the current in oneantenna with reference to that in the other by a suitable amount bymeans of the control means provided.

'When it is desired to transmit a powerful radiation eld to providecoverage over a distant territory, it is necessary to supply theantennas with considerable power as, for example, fifty kilowatts. Theradio signals to be transmitted are built up to such a high power levelby amplifying them in a plurality of vacuum tube amplifiers connected incascade, such series of amplifiers comprising several intermediateamplifiers which amplify the signal' at low power levels and driver andfinal amplifiers which utilize high` voltages and operate at high powerlevels. It is a characteristic feature of the present invention that thephase shift is secured by means of a device connected with the inputcircuit of one of the intermediate amplifiers where the power level islow. This arrangement contributes to the safety and convenience ofoperation of the system in that the operator of the control device,which effects the phase shift, is not exposed to the high voltage andcurrents which he otherwise would be ifV the phase shifting device wereconnected to one ofthe transmitting lines connecting the final highpower amplifier to its antenna. Furthermore, the desired arrangementreduces the cost of construction substantially over what it would be ifthe control device were operated at a high power level where its partswould necessarily be massive to carry the heavy currents and would alsohave to be highly insulated.

Further objects of the invention will become apparent as the descriptionthereof proceeds. For a better understanding of the invention referenceis made to the following description taken in connection with theaccompanying drawing, in which;

Fig. 1 is a schematic circuit arrangement of a directional radiotransmitting system embodying the invention; and

Fig. 2 is a map of a portion of the earth in oblique azimuthalprojection showing also approximate directional characteristic patternsadapted to be produced by the transmitter shown in Fig. 1.

Referring to the illustrative embodiment of the invention shown in Fig.1, I0 denotes a crystal controlled vacuum tube oscillator, I I anintermediate ampliiier, I2 a modulator in which the amplified carriercurrents are modulated in amplitude by the speech, music or othersignals tol be broadcast, and I3 an intermediate amplifier in which themodulated radio frequency signal cur--` rents are amplified. From theoutput of I3 the amplified signal currents are transmitted to a pair ofrhombic antennas I4, I 4 through the paths I5 and I6. The antennas I4,I4 are disposed parallel to each other at thel desired azimuth and arespaced apart a suitable distance from each other to secure the desiredresultant main radiation eld pattern. Each antenna is suitably supportedin a stationary position on poles and may be constructed to transmit alarge amount of power as, for example, 50 kilowatts.

Circuit pathf |15 is shownas comprising a variometer Il, an intermediateamplifier I8, a pair Variometer I'I is illustrated as comprising a xedwinding 22 and a movable winding 23 which` is rotated by a shaft 24provided with a control knob 25. Shaft 24 carriesa graduated dial 25, asuitable stationary index mark 21 being provided to indicate the dialreading. As illustrated dial 25 is preferably graduated in degrees oitrue bearing of the main resultant radiation eld of the two antennas I4and I4. For monitoring the separate elds radiated by the antennas, apair of monitor pickups 28, 28 are provided, one disposed in front ofeach antenna, the outputs of these pickups being transmitted to a phasemonitor 29 of known type by a line 3B, the monitor indicating the phasedifference between the'currents received on the pickups.

Assuming that it were desired to broadcastto either Buenos Aires or Riode Janeiro from a transmitter located at Scituate, Mass., the similarrhombic antennas would be mounted so that they were directed in thedirection OA, Fig. 2, to a region or point A midway between thesecities. Ii the several elements of circuit path I5 are exactly similarto those of circuit path I6 and antenna I4 is exactly similar in sizeand construction to antenna I4', no correction need be applied by thevariometer I1, since the currents supplied to amplifiers I8 and I8 willbe inV phase as will! also the currents in antennas I4 and I4'l so thatthe resultant main radiation iield of the two antennas will be directedexactlyat point A. If, however, the circuit paths I5Y and I5 are notexactly balanced as, for example, transmission line 2I being slightlydiierent in length or construction from transmission line 2|', orantenna I4 being slightly diierent in size or construction from antennaI4', thenl the signal currents in antennas I4 and I4 will not be inphase and the resultant main radiation field will not be directedexactly to point A but to a region on one side or the other of thispoint. This unbalance between the circuit paths I5 and I6 will beindicated on the phase monitor 29 and may be compensated for by theoperator adjusting the control knob of variometer Il. This adjustmentwill make the current supplied to amplifier IB lead or lag that suppliedto amplifierV I8 by the amount necessary to cause Athe azimuth of themain resultant radiation field to be moved in the direction oi point Auntil the desired coverage is secured. y

Ifrnow-'it is desired to broadcast to Buenos Aires in Spanish, thecontrol knob 25 is rotated inV such direction'for example, as to causethe current supplied to amplier I8 to lag that supplied to amplifier I8.This causes the current in antenne.' I4 to lag that in antenna |47', theamount Yof the lag being indicated on'the phase monitor 29 and .causesthe azimuthV of the resultant radiation eld to be shifted inthedirection of Buenos Aires. Such shifting may be continued until thedirectionhas been shifted `through an angle of A 4 the order of 10degrees before the two antenna elds get so much out of phase with eachother that they start to separate and no longer reinforce each other.'I'he phase shifting device shown thus provides means whereby theresultant main radiation field may be directed exactly in the directionof Buenos Aires as indicated by curve B, Fig. 2, and the desired regioncovered in a Spanish broadcast. The dial 26 of the Variometer may becalibrated, as shown, in degrees of true bearing of the resultantradiation eld so that the operator may readily direct the eld in thedesired direction or it could be calibrated in terms ofridegreesscanning of the radiation lield.

If it is later desired to transmit a broadcast in the Portugueselanguage to Rio de Janeiro, it is only necessary for the operator toadjust the control knob 25 in the opposite direction until the phase ofthe signal current supplied to amplifier I 8 leads that supplied toampliiier I8 by the necessary amount, as determined by the reading ofphase monitor 29. The resultant main radiation eld'will then be directedtowards Rio deJaneiro as illustrated by curve C, Fig. 2, and thebroadcast may proceed in the usual manner. While the phase shiftingdevice I'I has been illustrated as a variometer, it will be understoodthat a variable capacitive device or other known type of phase shiftingnetwork may be used instead.

I have described what I believe to be the best embodiments of myinvention. I do not wish, however, to be conned to the embodimentsshown, but what I desire to cover by Letters Pattent is set forth in theappended claims.

I claim:

l. A radio transmitting system comprising, in combination, two similarspaced rhombic antennas disposed at substantially the same azimuth, twocircuit paths connected to corresponding ends of said antennas and eachcomprising a plurality of vacuum tube ampliers connected in cascade, anda control device connected to the input of an amplifying stage precedingthe final stage of one path,V said control device being adjustable tovary the phase of the amplified signal current in one antenna withreference to the phase of the amplified signal current in the otherantenna during transmission to thereby vary the direction oftransmission of the resultant radiation eld of the two antennas andmeans connected to the first amplifying stages cf said circuit pathsforV supplying the signal currents thereto, said final amplifying stageoperating'at a high power level and the amplifying stage preceding thenal stage operating at a low power level.

2. The method of transmitting radio frequency signal currentssimultaneously from two spaced similar rhombic antennas disposed atsubstan- Y tially the same azimuth which comprises the steps `whileoperating the amplifying stage preceding saidfinal stage ata low powerlevel and operati--v high power level. Y

ing the nal stage at a Y Y MARK L. MACADAM.

